You’re right about the impossibility of separating good from bad ballots once they’re commingled for counting. Therefore, recounts are not useful.
The answer is to prove conclusively that the Dems violated state laws in place to ensure election integrity. Once that’s proven, a court can order a re-vote.
And a re-vote, under federal supervision, is also the best way to convince most voters that the final election results are valid.
Outside of a “re-vote” I see no other way besides putting it in the House.